1. Field of the Invention
The present invention provides a vapor deposition process for forming a passivation layer on a bare metal mass and/or surface, such as an aluminum mass and/or surface.
2. Brief Description of the Related Art
Currently, the most successful method for the production of metal nanoparticles is through a metal evaporation and inert gas condensation process. This method offers a high degree of control over the particle size and distribution through the gas selection, operating pressure, and gas flow rate. The disadvantage to this method in the use of metal nanoparticles, such as aluminum in energetic formulations, has been in the excessive relative amounts of oxide to metal required for surface passivation. Aluminum particles may be prepared by metal vapor condensation techniques or decomposition of AlH3—N(CH3)3 decomposition. These aluminum particles have been passivated by oxygen, with the oxygen forming a shell of aluminum oxide (Al2O3) over the core of aluminum or by adding the particles to a halogenated polymer slurry and allowing the polymer to set. Both of these methodologies allow oxygen to penetrate to the core of the particle and continue oxidation of the metal center with time and exposure to air. With the continued oxidation, the energy obtained during the combustion results in less than the theoretical maximum either from the incomplete combustion of the aluminum particle, i.e., the oxide layer prevents or retards combustion, or from a large amount of the aluminum, such as from 20% to 40%, being already fully oxidized prior to combustion. For example, U.S. Pat. No. 6,179,899 to Higa et al. discloses passivation of an aluminum powder product in the reaction vessel either by exposing the solution to air before product separation or by controlling the admission of air to the separated, dried powder.
There is a need in the art to provide an improved method for, and product of, passivated metal masses, particularly aluminum masses that contain a large amount of pure aluminum. The present invention addresses this and other needs.